1. Field of the Invention
This invention relates to packaging techniques for integrated circuits and , more particularly, to techniques for mounting various sizes of integrated-circuit dies to a common leadframe and to techniques for improving the thermal performance of an integrated-circuit package .
2. Prior Art
In a conventional semiconductor integrated-circuit package, electrical connections to the bonding pads of an integrated-circuit die are provided through a thin metal leadframe, which is typically stamped or chemically etched from strips of copper-containing materials. The integrated-circuit die is mounted to a centrally-located die-attach paddle, or pad, of the leadframe. The die-attach paddle is rectangular in shape and is supported at each of its four corners by a radially extending support beam. The leadframe includes a number of thin, closely-spaced conductive leads which radially extend away from the edges of the die. The leads diverge away from the die and extend through the exterior walls of the molded package where they form the external I/O leads for the package. The innermost end of the leads are called bonding fingers. Very thin gold wires are bonded at one end to the bonding pads on the die and at their others ends to the bonding fingers of the leadframe. To fabricate leadframes having closely spaced bonding fingers, chemical etching techniques are used. As the number of I/O leads increases and as die sizes shrink, the spacing between the bonding fingers of the leadframe decreases so that it is difficult to fabricate leadframes even using chemical etching when the required spacing between the bonding fingers approaches the thickness of the leadframe material itself.
Another problem with the number of I/O leads increasing and the die sizes shrinking is that the bonding fingers at the ends of the converging leads must be located at specified distances away from the die to maintain a minimum spacing between leads, so that the distance from the bonding pads on the die to the bonding fingers increases. If the length of the bonding wires exceed 150 mils, a fabrication problem called "wire-wash" might occur in which the long, thin, typically gold bonding wires may be swept together or broken by the flow of a plastic epoxy molding material as the die and the leadframe are encapsulated within the plastic epoxy molding material to form the body of the package. Consequently, bonding wire lengths are sometimes limited to approximately 150 mils to minimize this problem.
One solution to this wire-wash leadframe-connection problem is to provide intermediate "bridges" between the die and the bonding fingers so that the distance therebetween can be spanned by two shorter segments of bonding wires.
One such bridge is disclosed in U.S. Pat. No. 4,754,317 entitled "Integrated Circuit Die-To-Lead Frame Interconnection Assembly and Method" by Comstock et al. FIG. 1 of the drawings for the instant patent application shows a sectional view of a prior art integrated-circuit package assembly 10 which is similar to the Comstock 317 reference. On the top surface of an integrated-circuit die 12 are provided bonding pads 13 to which are attached bonding wires by techniques known in the integrated-circuit packaging art. The die 12 is conventionally mounted to a die-attach pad 14 which is part of a leadframe assembly for leads having bonding fingers 16, typically shown. The die 12 lies within a central cavity 17 formed within an insulated bridging member, or substrate assembly 18. The substrate assembly includes conductors formed on a relatively thick insulator layer 22. The conductors 20 serve as intermediate connection points for a typical first bonding wire 24, which spans the gap between the bonding finger 16 and the conductors 20 . The conductor 20 also serves an intermediate connection point for a typical second bonding wire 26, which spans the gap between one of the bonding pads 13 of the die 12 and one of the conductors 20. Encapsulating plastic epoxy material is molded around the die and the leadframe to form a body 30 for the integrated-circuit package assembly 10 . The technique of using an intermediate substrate assembly 18, which has conductors formed thereupon and which is placed between the outer periphery of a die and the inner periphery of the bonding fingers of a leadframe, reduces bonding-wire sag and bonding-wire wash. For each I/O connection, one wire is bonded between one end of a conductive pathway on the substrate assembly and the die and another wire is bonded between the other end of the conductive pathway on the substrate assembly and a bonding finger on the leadframe.
U.S. Pat. No. 4,774,635 entitled "Semiconductor Package With High Density I/O Lead Connection" by Greenberg et. al. discloses a semiconductor packaging technique. FIG. 2 of the drawings for the instant patent application shows a prior art integrated-circuit package assembly 40, similar to the Greenberg 635 structure, for an integrated-circuit die 42 . The die 42 has bonding pads 43 on its top surface, to which are attached bonding wires by techniques known in the integrated-circuit packaging art. The die 42 is conventionally mounted to a conventional die-attach pad 44 which is part of a leadframe assembly for the leads 4 6. The package assembly 40 includes an intermediate insulated layer 52, which has very thin, fragile conductive fingers 54 formed on its top surface. The conductive fingers 54 are bonded at one end to the bonding fingers 46 of the leadframe. The conductive fingers 54 shorten the gap which must be covered by bonding wires 60 extending from the bonding pads of the integrated-circuit die 42 . The die 42 sits within a cavity 62 formed within the center region of the insulated layer 52. Note that this technique requires precise alignment and assembly of the thin, fragile conductive fingers 54 to their respective bonding fingers 46, which requires expensive fabrication equipment and processing steps.
The structures described in the Comstock 317 patent and the Greenberg 635 patent are limited to being used with integrated-circuit dies having only a certain size because a die must to fit within the cavities provided at the center of the leadframe. Consequently, a number of different sizes of leadframes must be prefabricated to accommodate various die sizes .
Consequently, it is apparent that a need exists for an integrated-circuit packaging technique for mounting a number of different die sizes to either a standard leadframe or to a non-standard leadframe, which is commonly used with a number of different die sizes . To substantially reduce manufacturing costs, it is preferred that a standardized, stamped lead frame be available for a number of die sizes
Because of the rectangular geometry of a die and the converging pattern of the leads, the distance between a bonding pad and its corresponding bonding finger varies, depending upon whether the bonding pad is located near a corner of the die or near the midpoint of the side of the die. For example, the bonding distance for a bonding pad near a corner of a die may greatly exceed 150 mils, while the bonding distance for a bonding near the midpoint of a side may be approximately 150 mils. Consequently, a need exists for an integrated-circuit packaging technique which can optionally provide an intermediate bridge point for a long bonding wire span.
Sometimes it is necessary that an integrated-circuit die be electrically insulated from the die-attach pad to which the lower surface of the integrated-circuit die is attached. This may require the use of a dielectric material between the lower surface of the die and the conductive die-attach pad. Smaller dies typically require better thermal dissipation characteristics from their packaging configuration. Consequently, the need exists for an integrated-circuit technique which permits a die to be electrically insulated, but thermally connected to a die-attach pad portion of a leadframe.